(1) Field of the Invention
The present invention relates to a method for producing an aqueous solution containing a metal at a high concentration, particularly to a method for producing an aqueous solution containing titanium at a high concentration by using at least one of ammonia, primary, and secondary and tertiary amines.
(2) Description of the Related Art
It is known that solutions containing titanium are produced by dissolving titanium alkoxides into various organic solvents such as alcohols or hydrocarbons. The solutions produced by such a method are being used for producing photocatalyst and dielectric thin films of composite oxides.
There are known peroxytitanium aqueous solution prepared by a reaction between so-called titanic acid obtained through hydrolysis of titanium tetrachloride or a titanium alkoxide and hydrogen peroxide or a reaction among metallic titanium, hydrogen peroxide, ammonia water and hydroxycarboxylic acid. Particularly, the complex obtained by the latter reaction is known as xe2x80x9cTAS FINExe2x80x9d (commercial name).
However, since the alkoxide has a high reactivity with water, the titanium-containing solution obtained by using one of the above organic solvents needs to be prevented from contacting with moisture in air during the synthesis. The resulting aqueous solution is unstable with water, so that it must be prevented from contacting moisture during storage. Furthermore, since the organic solvent used is inflammable, attention needs to be paid to fire. Since the organic solvent is used, the solution may ignite through heat generation during the hydrolysis.
Further, such a solution is not suitable as a starting material for composite oxides containing alkali ions or alkaline earth metal ions (for example, barium titanate or potassium titanyl phosphate (KTP)). Since titanium compounds are unstable with water, most of them are used together with solvents having high environmental loads, such as organic solvents. However, the above-mentioned problem still exists. Investigations have been earnestly made to remove as much environmental load-increasing factors as possible from steps in the production and also to cause no pollution. How to use factors causing low environmental loads is a problem.
The above problem can be solved by converting the titanium compound to an aqueous solution. However, there is a tendency that an aqueous solution having ions with high positive valency (+4 or more) easily causes aqua-complex ions to be hydrolyzed and condensed, and generally causes precipitation of a basic oxide thereof. The titanium ion is one of typical ions exhibiting such a behavior. Therefore, the titanium-containing aqueous solution is not obtained unless a highly acidic condition is applied.
The above-mentioned peroxytitanium solution is known as only one example. However, this solution has the drawbacks that it contains chlorine even if the concentrations are low when titanium tetrachloride was used as the starting material and/or the concentration of titanium contained in the aqueous solution is low unless stabilized by a suitable chelating agent. The concentrated solutions may contain a hydroxycarboxylate as a ligand, for example. Therefore, an aqueous solution containing titanium at a high concentration is desired, which contains simple titanic acid ions only and does not cause gellation or precipitation over a long time period. Since the solution proposed in this invention, does not contain a complicated ligand with a large molecular weight, it is an optimum starting material for the synthesis of derivatives from titanic acid. However, such a titanium-containing aqueous solution or its preparation method has not been known.
It is an object of the present invention to provide an aqueous solution containing titanium at a high concentration, with excellent homogeneity and stability over an extended time of period.
In order to realize the above object, the present inventors reacted titanium alkoxides with water in the presence of amines, etc. As a result, the inventors discovered the method for producing the titanium-containing aqueous solution according to the present invention.
The method for producing the titanium-containing aqueous solutions according to the present invention is characterized by reacting a titanium alkoxide with water in the presence of at least one of ammonia, and amines selected from a primary amine, a secondary amine, and a tertiary amine (hereinafter, these are also referred to xe2x80x9camines, etc.xe2x80x9d or xe2x80x9camines or the likexe2x80x9d, provided that when ammonia and/or the primary amine only is used, at least one selected from the group consisting of an xcex1-diketone, a xcex2-diketone, an xcex1 hydroxyketone and a carboxylic acid is used in combination.
As a preferred embodiment of the method for producing the titanium-containing aqueous solutions according to the present invention, the amines, etc. comprise at least one selected from the group consisting of N-methyldiethanolamine, N-ethyldiethanolamine, diisopropanolamine, triethanolamine, dimethylamine, trimethylamine, diethylamine, and triethylamine.
As another preferred embodiment of the method for producing the titanium-containing aqueous solutions according to the present invention, the reaction is carried out in the presence of the ketone or the like.
The xe2x80x9cketone or the likexe2x80x9d include at least one selected from the group consisting of an xcex1-diketone, a xcex2-diketone, an xcex1 hydroxyketone and a carboxylic acid.